This invention relates to drop on demand ink jet printing apparatus and more particularly to a valve and nozzle system for suck apparatus.
Drop on demand ink jet printing apparatus operates to discharge individual droplets of ink onto a substrate in a predetermined pattern to be printed. Such apparatus incorporates an array of orifices in a nozzle block, a plurality of control valves, and a controller. The orifices are customarily arranged in a vertical row, and conventional ink jet printing apparatus has incorporated a separate valve communicating with each orifice. The valves are controlled by the controller, which can be keyed by an operator to open and close the nozzles according to a programmed schedule. Thus the operator can establish controller instructions to cause the valves to operate in irregular sequences and at irregular intervals to control the flow of ink through the respective orifices to print one or a series of desired characters or symbols.
Each orifice is designed to emit a single droplet of ink during each opening of its associated valve. The droplets, emitted according to the programmed sequence, are directed toward a substrate where the character or symbol is printed. The quality of print produced by a drop on demand ink jet printer requires precise control over the size of the ink dot that impacts the substrate. Dot size in turn is affected by the size of an ink droplet discharged from a nozzle.
It is critical in the overall design represented by the relationship between valve characteristics, orifice size, and ink characteristics, that the droplets not only be of proper size but also that the size be consistent because otherwise the printed characters or symbols will be irregular in width.
The size of a drop of ink is influenced by the characteristics of the valve and the orifice as well as by the ink specifications, most notably viscosity. For a given ink and a given orifice diameter, the duration of the open period of the valve will affect the size of the drop. Since this open period is very short for an ink jet valve, it is important that the cycle begin with abrupt opening and end with abrupt closing of the valve.
Also, in drop on demand ink jet printing, the substrate is usually moving, may have a wavy surface causing variations in distance the ink droplets must travel before reaching the substrate, and is of varieties of cardboard compositions and consistencies affecting wicking of the ink, to name a few significant variables. Considering these variables, it can be appreciated that great care must be given to a change in any of the components of the system. Thus, the need for uniformity and accuracy requires that all of the components, including the valve, be manufactured with precision. Also, since the valve is relatively expensive and must operate in a great number of short open and close cycles, it must retain its precision through many millions of cycles.
Typically, in early ink jet printing apparatus, a nozzle orifice array consisted of a vertical row of seven orifices coupled with seven control valves. Each control valve controlled the flow of ink through its associated orifice. An example of such a drop on demand ink jet printing apparatus is described and illustrated in U.S. Pat. No. 4,378,564. The subject matter disclosed by that patent is incorporated herein by reference.
In time, the need developed for an increased number of orifices. To meet this need, a larger number of orifices were assembled in a taller vertical array, and a correspondingly greater number of valves were incorporated, again, each nozzle orifice having its own control valve. The typical approach was to increase the number of orifices by superimposing two or more orifice nozzle arrays, each array incorporating the same number of valves as orifices. The increased number of valves increases the cost of the printing apparatus.
A factor in the operation of this equipment is surface tension of the ink. There is a tube connecting each valve outlet port with its associated orifice. When the valve closes, but for surface tension at the orifice opening, ink in the tube would continue to flow through the orifice and destroy the droplet. This surface tension, resulting from viscosity of the ink and the diameter of the orifice, resists the ink pressure upstream of the orifice.
This surface tension at the orifice opening holds ink within the tubing between a valve and an orifice after the valve closes. Without the surface tension, upon closing of the valve upstream of the tubing, ink would drain from the tubing through the orifice. Such surface tension would be lost, for example, if the tubing upstream of the orifice was exposed to the atmosphere and if the strength of the surface tension could not counteract atmospheric pressure. Also, head pressure differentials do not exist at the orifices if the tubes are unexposed to atmospheric pressure. Because of the surface tension, the flow of ink stops immediately when the valve closes. When the valve opens again, a droplet instantly begins to form and, because of the ink source pressure, the droplet is completed and discharged from the orifice in the short time the valve is open.
As has been said, in order for the valve to maintain its precision of operation over many millions of cycles of opening and closing, the design of the valve is crucial. In the conventional ink jet printing system, each valve is solenoid operated and has an ink chamber with a single inlet port and a single outlet port communicating with the chamber. A piston face is actuable against a valve seat surrounding the outlet port to open and close the valve. In this valve, the chamber is large enough to accommodate a piston head having a smaller stem of magnetically responsive metal so that the stem can function as the core of a solenoid. A compression spring normally holds a face of the piston head in contact with the outlet port seat to close the valve. When the valve is closed, the inlet port remains in communication with the chamber. When the solenoid is energized, its magnetic field overcomes the strength of the compression spring and withdraws the piston head from the outlet port, allowing ink to flow from the inlet port, through the chamber to the outlet port. When the magnetic field is released, the compression spring drives the piston head back to close the outlet port.
In ink jet printing done heretofore, the spacing between orifices has produced a printed character or symbol composed of essentially discrete dots of ink. Because of the number of them, these discrete dots have been acceptable in producing a readable character or symbol. However, the traditional ink jet printing apparatus has not been acceptable to print bar codes because of the specifications for bar code printing required to assure accurate reading of the bar codes.
In a bar code, there are a plurality of vertical bars of different widths, and the blank spaces between adjacent bars are of different widths. Bar codes are read by reading the widths of both the printed lines and of the blank spaces between adjacent printed lines. Thus, to read such a bar code accurately, the sequence of varying widths of printed bars and intervening spaces must be read accurately. Accordingly, the widths of the bar codes must be uniform from top to bottom within specification tolerances. These specification tolerances are not met by the wavy side edges of lines that are formed by discrete printed dots produced by conventional ink jet printing apparatus.
To eliminate the waves on the side edges of a printed line or "smooth out" the composite side edges of a resulting printed bar, the printed dots must overlap oneanother. The conventional way to accomplish this would be to produce a nozzle assembly having a large number of orifices in a vertical row positioned very close to one another so their images, after wicking, would overlap oneanother, and to provide a correspondingly large number of control valves. As conventional, each orifice would be under the control of an undivided valve connected to it. This addition of valves would add to the cost of the ink jet printing apparatus and to the volume occupied by them.
In the present invention, a nozzle block has an array of orifices that are close enough together to smooth out the side edges of a printed vertical line or bar. For this purpose, it has been determined that the distance between orifice centers is substantially one-half the diameter of the dot as printed. For example, assuming the dots size is to be about 0.040 inch in diameter, the centers of adjacent orifices are spaced by about 0.020 inch. To produce the overlapping printed dots requires and increased number of orifices, 64 in the preferred embodiment. However, pursuant to this invention, far fewer than 64 valves are required.
In the valve of the present invention, there is an ink chamber. A single inlet port to the chamber communicates with a source of ink under predetermined pressure. There are a plurality of outlets ports also communicating with the chamber. A piston is operable within the chamber to alternately simultaneously block and simultaneously unblock all outlet ports. Each outlet port is connected by tubing to an individual orifice, but since there are a plurality of outlet ports, a single valve controls the flow of ink through a corresponding plurality of orifices. Surface tension can be maintained at each orifice opening so that at the instant the piston closes the outlet ports, the flow of ink stops and, upon withdrawal of the piston from the outlet ports, ink instantly flows to all of the orifices where ink droplets are formed and discharged.